PCT/EP2013/074796 describes a cover device for a drink container comprising a basic assembly which is provided with a drink opening and a valve arrangement for blocking or unblocking a passage to the drink opening from a drink container side of the cover device. The valve arrangement comprises a valve element having two portions which are connected to each other through an area of the valve element at a position where the valve element is hingably associated with the basic assembly, and wherein only one of the two portions is in direct communication with the drink opening of the basic assembly, so that a smallest total moment of force may be realized on the one portion when the valve element is subjected to pressure from the drink container side of the cover device. In the central portion a slit is present which will open upon suction force.
The thin material of the valve element at the location of the slit deforms when overpressure is present in the drink container on the valve element, sometimes resulting in opening of the slit. Overpressure occurs e.g. when the drink container is held upside down or is shaken.